This invention relates to apparatus and methods to thermally process substrates.
In many semiconductor-manufacturing processes, substrates are thermally processed in a series of one or more phases. For example, some thermal processes include a pre-heating phase during which the substrate is heated to an initial temperature before the substrate is loaded completely into a processing chamber and processed with a prescribed heating cycle. To achieve the required device performance, yield, and process repeatability, the temperature of a substrate such as a semiconductor wafer is strictly controlled during processing. For example, semiconductor devices have layers that are tens of angstroms thick and this thickness uniformity must be held to within a few percent. Potential problems arising from a non-uniform substrate temperature include semiconductor crystal slips that can destroy devices through which the slip passes. Additionally, certain semiconductor processes, such as those to form an epitaxial layer, require a uniform temperature to obtain uniform resistivity. These requirements dictate that temperature variations across the substrate or wafer during processing be limited to a tight range.
To achieve the desired substrate temperature, certain process chambers use one or more high intensity heating elements, such as lamps, positioned over the substrate to be heated. Potential problems with the use of high intensity lamps as a heat source, particularly for larger diameter wafers include difficulties in maintaining a uniform temperature across the wafer. Further, temperature differences can arise during heating/cooling transients and during processing. The interior walls of typical lamp based systems are usually relatively cool and are not heated to a uniform equilibrium process temperature as in a conventional batch furnace. Different radial locations on the wafer surface receive different fractions of their incident radiation from each of the lamps and have different views of the relatively cool side walls. As a result, it may be difficult to ensure that the net radiant heat flux, and hence the equilibrium temperature may not be uniformly maintained on the wafer.